1. Field of the Invention
This invention relates generally to demagnetizing magnetic recording media, and, more specifically, to an apparatus and method for bulk erasure of disk drives.
2. Background Information
It is often desirable to imprint a magnetic pattern onto a medium (e.g., a disk, a tape, a credit card, etc.) in a predictable and reproducible manner. One approach to imprinting a magnetic pattern onto a medium is servo media printing. In particular, a conventional magnetic recording disk typically contains servo patterns written in preselected regions (“servo sectors”) on the disk surface. The servo patterns provide position and tracking information during a readout process. For instance, each servo pattern may store information that enables a read head to determine its relative position on the disk surface. In addition, the servo pattern also may be configured to cause the read head to generate a position error signal when it reads the servo pattern. The position error signal is then fed back to a controller that controls the head's tracking.
The servo patterns are typically written on the magnetic recording disk during a disk-manufacturing process, e.g., in a clean room environment. There are various known techniques for writing the servo patterns on the disk. One common technique is to program a magnetic write head to write the servo patterns as the disk rotates beneath the head. For instance, some disk drives are capable of self-servo writing, such as, e.g., by “spiral writing” the servo pattern on the disks, as will be understood by those skilled in the art.
Another known technique for writing servo patterns utilizes magnetic printing. A printing master is constructed having a plurality of ferromagnetic “teeth” that are constructed and arranged to coincide with at least one servo pattern. The teeth may be used either to shield their contacted disk surfaces from the applied writing field (e.g., longitudinal recording), or alternatively to concentrate magnetic flux at their contacted surfaces (e.g., perpendicular recording). In either case, a copy of the teeth pattern is written onto the disk and the printing master is removed.
Yet another known technique for writing servo patterns employs laser light to transfer the patterns to disk using heat assisted magnetic recording (HAMR). Incident light is focused at selected areas on the disk surface to produce a pattern of heated areas corresponding to the servo patterns, and an external magnetic field is applied to the heated areas to copy the servo patterns to the disk.
For many (if not all) of these servo writing techniques to work properly, the disk should be substantially free of magnetic signals (i.e., a “clean” disk). For instance, during manufacture, a recording layer often develops various magnetized regions that may cause magnetic interference during servo-write operations. These magnetic regions may result in track profile asymmetries. For recording systems that use signals from prewritten tracks or sector servos to position the read head, such asymmetries can result in positioning errors. In particular, when first servo writing a disk surface, these magnetic regions, with generally unknown locations, may adversely effect the precision with which the servo tracks are written (e.g., especially for self-servo writing drives). Therefore, “preconditioning” the media (e.g., disk) to reduce the large magnetized regions becomes very important. Also, during various stages of manufacturing the disk drives, the disks may need to be “reworked” (preconditioned again, or re-initialized, etc.).
“Bulk erasing” may be used to erase any undesired (and unknown) magnetic regions, or domain walls, on the disk surfaces, such as pre-magnetizing the disk in a uniform direction. One method used to bulk erase a disk is a direct current (DC) erase method using a write head of the disk drive. In particular, the write head passes over the entire disk (i.e., every track of the disk) and directs the magnetizations in a single direction to erase the disk. Those skilled in the art, however, will understand that because write heads are generally small in size and designed to write a single track at a time, this method is time consuming, as it requires the write head to pass over every track of the disk (e.g., 40 minutes).
Alternatively, a faster method to bulk erase a disk utilizes an “open loop” external magnetic field across the entire disk surface (i.e., from outside diameter (OD) to inside diameter (ID)). For instance, two large permanent magnets may be spaced a certain distance from one another in order to create a large DC magnetic field between them. The entire disk drive (e.g., within a cover or removed from the cover) is then inserted into the DC field to erase the disks contained therein. One problem associated with using large permanent magnets in this manner is that the erasure may not be precise, and may cause stray magnetic fields to reach components of the disk drive other than the disks themselves. Particularly, these stray, and often very strong magnetic fields, may cause damage to sensitive components, such as partially demagnetizing motor magnets of the drive. For example, typical motor magnets may be demagnetized with a field greater than 2 kilo Gauss (kG). However, to erase a disk, approximately 5 kG must reach the surface of the disk, so stray fields of 2 kG or more (e.g., up to the erasing field, 5 kG) may also inadvertently reach the motor magnets, thus causing damage. Further, another problem associated with using permanent magnets in this manner is that they are often big and bulky designs, and having large permanent magnets in a manufacturing clean room, even with adequate shielding (also big and bulky), may cause undesirable magnetic contamination to other devices in the room.
There remains a need, therefore, for a system that efficiently (e.g., quickly and completely) bulk erases the surface of a disk in a disk drive, without causing potential harm to sensitive components of the disk drive. Further, there remains a need for such a system that may apply DC and alternating current (AC) erasing to be effectively operational for both longitudinal recording media and perpendicular recording media.